Servo muscle control

ABSTRACT

A method of and apparatus for controlling one or more parameters of an electrical stimulation generator in response to measured results of the stimulation. In the preferred mode, this technique is employed in a system for the treatment of obstructive sleep apnea. Sensors are used to determine the effectiveness of the stimulation. Amplitude and pulse width are modified in response to the measurements from the sensors.

This is a continuation of co-pending application Ser. No. 07/639,192filed on Jan. 9, 1991 now abandoned.

CROSS REFERENCE TO CO-PENDING APPLICATIONS

U.S. patent application Ser. No. 07/610,854, filed Nov. 8, 1990, nowU.S. Pat. No. 5,133,354, and entitled "Improving Muscle Tone"; and U.S.patent application Ser. No. 07/617,158, filed Nov. 23, 1990, nowabandoned and entitled "Multiple Stimulation Electrodes" are bothassigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electrical stimulation ofmuscles, and more particularly, relates to electrical stimulation ofmuscles for the treatment of a medical condition.

2. Description of the Prior Art

It has been known to electrically stimulate muscular contractions sincethe beginnings of experimentation with electricity. In more recenttimes, electrical stimulation of muscle tissue has been usedtherapeutically. The effects of chronic stimulation have been studied byCiske and Faulkner in "Chronic Electrical Stimulation of Nongrafted andGrafted Skeletal Muscles in Rats", in Journal of Applied Physiology,Volume 59(5), pp. 1434-1439 (1985). Bernotas et al., have even suggestedthe rudiments of adaptive control in "Adaptive Control of ElectricallyStimulated Muscle", in IEEE Transactions on Biomedical Engineering,Volume BME-34, No. 2, pp. 140-147, (February, 1987).

A review of early attempts at electrical stimulation associated with therespiratory system is found in "Diaphragm Pacing: Present Status", byWilliam W. L. Glenn, in Pace, Volume 1, pp. 357-370, (July-September,1978). Much work has been done in electrical stimulation within thecardiovascular system by way of cardiac pacing.

Treatment of obstructive sleep apnea using electrical stimulation hasalso been discussed. "Laryngeal Pacemaker, II Electronic Pacing ofReinnervated Posterior Cricoarytenoid Muscles in the Canine", byBroniatowski et al, in Laryngoscope, Volume 95, pp. 1194-1198 (October,1985); "Assessment of Muscle Action on Upper Airway Stability inAnesthetized Dogs", by Strohl et al., in Journal of Laboratory ClinicalMedicine, Volume 110, pp. 221-301, (1987); U.S. Pat. No. 4,830,008issued to Meer; and U.S. Pat. No. 4,570,631 issued to Durkan all discusselectrical stimulation of the upper airway to treat obstructive sleepapnea.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byproviding a system for the electrical stimulation of muscle tissue,which is adaptive in nature. Sensors are employed within the stimulationsystem to modify parameters such as stimulation pulse amplitude andpulse frequency in response to the sensed performance of the therapydesired.

In the preferred mode, the present invention is applied to the treatmentof obstructive sleep apnea. Sensors are employed to determine thepressure differential between the distal pharynx and the ambient todetermine the pressure drop across the upper airway. Stimulationintensity is increased as the relative pressure differential increases,and decreased as the relative pressure differential decreases. Intensitymay be increased by increased pulse frequency and/or pulse amplitude.Additional sensing is required to accommodate the respiration cycle,permitting the system to adapt independent of the normal cyclicvariations.

There are a number of important results attendant to the adaptivesystem. The total energy required is lessened, because the stimulationintensity is maintained at only that level necessary to sustain thedesired clinical performance. The risk of muscle fatigue is greatlyreduced because the muscles of the upper airway are not over stimulated.Similarly, the lack of over stimulation provides the patient with a moreeasily tolerated therapy, particularly when used chronically.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects of the present invention and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, in which like reference numerals designate like partsthroughout the figures thereof and wherein:

FIG. 1 is a schematic diagram of the normal function of the respirationsystem of a patient during inspiration;

FIG. 2 is schematic diagram of the same patient with obstructive sleepapnea;

FIG. 3 is a graphical representation of airway conductance versusstimulation level in a patient with obstructive sleep apnea;

FIG. 4 is a schematic diagram of a patient having an electricalstimulation system employing the present invention;

FIG. 5 is a plan view of a chronically implantable pressure sensor;

FIG. 6 is a block diagram of an implantable pulse generator according tothe present invention; and,

FIG. 7 is a graphical representation of the key signals within theimplanted electrical stimulation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of the respiratory system of patient 10during inspiration. As a result of diaphragm 18 increasing the volume ofthorax 16, a pressure differential is created causing air to enter upperairway 12 and proceed in the direction of arrow 14.

FIG. 2 is a schematic diagram of the respiratory system of patient 10during an obstructive apnea event. During inspiration, upper airway 12tends to collapse producing the obstruction to air flow at point 21. Theabove-referenced literature describes in detail the physiologicalprocesses associated with the collapse of upper airway 12.

FIG. 3 is a graphical representation 52 of airway conductance 54 as afunction of stimulation frequency 56 in patient 10 suffering fromobstructive sleep apnea. At single twitch (57) or relatively low levelstimulation frequency 59, airway conductance 54 is insufficient as shownalong the corresponding portions of the curve. Single twitch and lowfrequency stimulation do not sustain conductance change (i.e.,sufficient tension) and result in tension plateaus which are too low tomaintain an open airway. Normal patency is airway conductance betweenlevels 64 and 66. To achieve normal patency, stimulation frequency mustbe at least as point 61.

For stimulation frequency greater than that for normal patency (i.e.airway conductance 54 of greater than level 64), very little improvementcan be observed. Portion 60 of the curve represents this state. As thesystem proceeds along portion 60, stimulation frequency is excessiveresulting in wasted energy and increased risk of muscle fatigue.

FIG. 4 is a schematic diagram of patient 10 showing implantation of anelectrical stimulation system for the treatment of obstructive sleepapnea. Implantable pulse generator 20 is placed subcutaneously at aconvenient position. The operation of diaphragm 18 is monitored byelectrode 24 coupled to lead 22.

Patency of upper airway 12 is monitored by pressure sensor 36 andpressure sensor 30 coupled to implantable pulse generator 20 via cables34 and 32, respectively. Stimulation of the musculature of upper airway12 is accomplished via lead 26 coupled to electrode 28. All otherreferenced elements are as previously described.

FIG. 5 is a plan view of a chronically implantable pressure transducer40 similar to that implanted as pressure sensors 30 and 36 (see alsoFIG. 4). Distal end 42 of chronically implantable pressure transducer 40contains a semiconductor sensing element properly package for chronicimplantation. Lead body 44 optionally contains pressure reference lumen46, which is coupled to pressure vent 48. Electrical connector 50couples to implantable pulse generator 20. For additional constructiondetails, the reader may consult U.S. Pat. No. 4,407,296 issued toAnderson incorporated herein by reference.

FIG. 6 is a block diagram of implantable pulse generator 20 made inaccordance with the present invention. Cables 32 and 34, coupled topressure sensors 30 and 36, respectively, provide the inputs todifferential amplifier 68. The output of differential amplifier 68 isthus representative of the pressure differential between the pharynx andthe mouth (see also FIG. 4).

The pressure difference signal is integrated by integrator 70 to providea smooth signal. The signal is clipped by clipper circuit 72 to scalethe signal. Detector 74 is a thresholding device. The output of detector74 is essentially a binary feedback signal 76 indicative of whetherupper airway 12 has sufficient patency. Binary feedback signal 76 isused to control pulse frequency control 78. In this way, the pulsefrequency of the stimulation pulses is continually increased untilsufficient patency is monitored. Pulse frequency control 78 may alsochange output amplitude of the stimulation pulses. The stimulationpulses are produced by stimulation generator 80, amplified by amplifier82, and coupled to the upper airway musculature by amplifier 82.

The stimulation pulses generated are timed in accordance with therespiration cycle by timing 84. This circuit also notifies clippercircuit 72 of the time window in which patient 10 is within aninspiration cycle. Timing 84 operates by drive from oscillator 86, whichis the main timing standard within implantable pulse generator 20.

The position within the respiration cycle is monitored by electrode 24coupled to lead 22 (see also FIG. 4). Other means can also be used tosense inspiration, such as impedance plethysmography. In the preferredmode, it is the EMG which is actually sensed. The EMG is amplified byamplifier 90 and integrated by integrator 92. This smooths the signalconsiderably. The integrated signal is supplied to trigger 88, which isa thresholded monostable multivibrator. The output of trigger 88 is afixed length signal having a leading edge occurring at the initiation ofan inspiration cycle. As explained above, timing 84 synchronizes theoutput of trigger 88 with the output of oscillator 86 and provides timewindows to clipper circuit 72 and stimulation generator 80.

FIG. 7 is a graphical representation 94 of various key signals ofimplantable pulse generator 20. Curve 96 shows the pressuredifferentials to be monitored for two respiratory cycles wherein thefirst cycle involves a substantial obstruction within upper airway 12,and the second cycle shows normal patency as a result of electricalstimulation of sufficient intensity. Portion 98 of curve 96 is thepressure differential resulting from inspiration with an obstructedupper airway.

Pressure differential 106 follows null period 104. Pressure differential106 shows inspiration under normal patency because the stimulationintensity has been increased.

Pulses 112 and 114 are the output of trigger 88 (see also FIG. 6). Theyprovide the timing window associated with the inspiration portion of therespiratory cycle. Stimulation pulses 116, 118, 120, 122, 124, and 126are supplied during the first inspiration. As seen above, the frequencyof these stimulation pulses is insufficient to produce normal patency ofthe upper airway. The feedback system ensures that the pulse frequencyof succeeding stimulation pulses 128, 129, 130, 131, 132, 133, 134, 135,136, 137, 138, and 139 is greater. In this case, the stimulationfrequency is sufficient to produce normal patency as is seen in pressuredifferential 106.

Curve 140 shows the output of integrator 92 (see also FIG. 6). It isfrom curve 140 that trigger 88 generates pulses 112 and 114. Peaks 142and 152 correspond to the inspiration periods. Similarly, negative peaks146 and 156 correspond to the expiration periods. Null periods 144, 148,and 154 separate portions of the respiratory cycle. Note that slope 150will be effected by the increase in stimulation intensity fromrespiratory cycle one to respiratory cycle two.

Having thus described the preferred embodiments of the presentinvention, those of skill in the art will readily appreciate the othervariations possible within the teachings found herein and within thescope of the claims hereto attached.

I claim:
 1. A method of treating obstructive sleep apnea in a patientcomprising:a. monitoring the respiratory cycle of said patient todetermine the inspiration portion; b. generating stimulation pulsescorresponding with said inspiration portion of said respiration cycle;c. monitoring upper airway patency during said inspiration portion ofsaid respiration cycle; and, d. adjusting the intensity of saidstimulation pulses in response to the results of said upper airwaypatency monitoring step to maintain normal patency of said upper airway.2. A method according to claim 1 wherein said monitoring upper airwaypatency further comprises measuring pressure differential during saidinspiration portion of said respiratory cycle.
 3. A method according toclaim 2 wherein said adjusting further comprises changing pulsefrequency of said stimulation pulses.
 4. A method according to claim 2wherein said adjusting further comprises changing amplitude of saidstimulation pulses.
 5. A method according to claim 3 wherein saidadjusting further comprises changing amplitude of said stimulationpulses.
 6. An apparatus for treating obstructive sleep apneacomprising:a. means for generating signals to stimulate upper airwaymuscle tissue of a patient; b. means coupled to said generating meansfor timing said signals to produce stimulation of said upper airwaymuscle tissue during inspiration by said patient; c. means formonitoring efficacy of stimulation of said signals; and, d. meanscoupled to said monitoring means and said generating means for adjustingintensity of said signals to efficiently increase patency of said upperairway of said patient.
 7. An apparatus according to claim 6 whereinsaid monitoring means comprises means for measuring temperaturedifferential across said upper airway of said patient.
 8. An apparatusaccording to claim 6 wherein said adjusting means comprises means forchanging pulse frequency of said signals.
 9. An apparatus according toclaim 7 wherein said adjusting means comprises means for changing pulsefrequency of said signals.
 10. An apparatus according to claim 6 whereinsaid adjusting means comprises means for changing amplitude of saidsignals.
 11. An apparatus according to claim 7 wherein said adjustingmeans comprises means for changing amplitude of said signals.